Hydroxide ion is the strongest base possible in water (aqueous) solutions, but superbases are much stronger than water-based bases. Such bases are useful in organic synthesis and are fundamental to physical organic chemistry. Superbases have been described and used since the 1850s. Reactions involving superbases often require special techniques since they are destroyed by water, carbon dioxide, and oxygen in the air. Inert atmosphere techniques and low temperatures minimize these side reactions.
IUPAC defines superbases simply as a "compound having a very high basicity, such as lithium diisopropylamide." Caubère defines superbases qualitatively but more precisely: "The term superbases should only be applied to bases resulting from a mixing of two (or more) bases leading to new basic species possessing inherent new properties. The term superbase does not mean a base is thermodynamically and/or kinetically stronger than another, instead it means that a basic reagent is created by combining the characteristics of several different bases."
Superbases have also been defined in terms of numbers (semi-quantitatively) as any species with a higher absolute proton affinity (APA = 245.3 kcal/mol) and intrinsic gas phase basicity (GB = 239 kcal/mol) than Alder's canonical proton sponge (1,8-bis-(dimethylamino)-naphthalene).
The "strength" of bases is measured by pKb, the "dissociation constant" of each base. Historically, the equilibrium constant Kb for a base has been defined as the association constant for protonation of the base, B, to form the conjugate acid, HB+.
- B + H2O HB+ + OH−
So, the equilibrium of this reaction is:
and pKb is defined as:
- pKb = − log10 Kb. The range for aqueous solutions (pH) is 1 to 14. But superbases are outside this range.
Classes of superbases [change]
Organic superbases are almost always neutral, nitrogen-containing species. Despite enormous proton affinity, chemists like organosuperbases for their high reactivity tempered by low nucleophilicity and relatively mild conditions of use. Increasingly important in organic synthesis, these include the phosphazenes, amidines and guanidines. Other organic compounds also meet the physicochemical or structural definitions of 'superbase'. Proton chelators like the aromatic proton sponges and the bispidines are also superbases. Multicyclic polyamines, like DABCO might also be loosely included in this category.
Organometallic compounds of reactive metals are often superbases, including organolithium and organomagnesium (Grignard reagent) compounds. Another type of organic superbase has a reactive metal exchanged for a hydrogen on a heteroatom, such as oxygen (unstabilized alkoxides) or nitrogen (metal amides such as lithium diisopropylamide). A desirable property in many cases is low nucleophilicity, i.e. a non-nucleophilic base. Unhindered alkyllithiums, for example, cannot be used with electrophiles such as carbonyl groups, because they attack the electrophiles as nucleophiles.
In organic synthesis, the Schlosser base (or Lochmann-Schlosser base), the combination of n-butyllithium and potassium tert-butoxide, is a commonly used superbase. n-Butyllithium and potassium tert-butoxide form a mixed aggregate of greater reactivity than either reagent alone and with distinctly different properties in comparison to tert-butylpotassium.
Inorganic superbases are typically salt-like compounds with small, highly charged anions, e.g. lithium nitride. Alkali lanthanides and alkali metal hydrides potassium hydride and sodium hydride are superbases. Such species are insoluble in all solvents owing to the strong cation-anion interactions. But the surfaces of these materials are highly reactive and slurries are useful in synthesis.
Other pages [change]
- "BBC - h2g2 - History of Chemistry - Acids and Bases". http://www.bbc.co.uk/dna/h2g2/alabaster/A708257. Retrieved 2009-08-30.
- "IUPAC Gold Book - superacid". http://goldbook.iupac.org/S06135.html. Retrieved 2009-08-30.
- Caubère, P. (1993) Unimetal super bases Chemical Reviews, 93, 2317-2334. doi:10.1021/cr00022a012
- Raczynska, E. D., Decouzon, M., Gal, J.-F. et al.(1998) Superbases and superacids in the gas phase. Trends in Organic Chemistry, 7, 95-103.
- Superbases for Organic Synthesis Ed. Ishikawa, T., John Wiley and Sons, Ltd.: West Sussex, UK. 2009.
- Schlosser, M. (1988). Superbases for organic synthesis, Pure and Applied Chemistry, 60(11), 1627-1634. doi:10.1351/pac198860111627